1. Field of the Invention
The present invention relates generally to nitride-based compound semiconductors, methods of cleaning compound semiconductors, methods of producing the same and substrates, and particularly to nitride-based compound semiconductors, methods of cleaning compound semiconductors, methods of producing the same and substrates, that can provide excellent cleanness on a surface having been cleaned.
2. Description of the Background Art
Nitride-based compound semiconductor, one example of compound conductors, has a large bandgap and is of direct transition type. As such, it is in practical use in light emitting diodes, semiconductor lasers and the like and is also being studied for application to power devices and the like. These devices employ a structure including a substrate and thin, nitride-based compound semiconductor films deposited on the substrate in layers.
Such thin, nitride-based compound semiconductor films are deposited on a substrate formed for example of gallium nitride (GaN) as well as sapphire, silicon carbide (SiC), silicon (Si), zinc oxide (ZnO), gallium arsenide (GaAs) and the like, and in the method of producing the nitride-based compound semiconductor, providing thin, nitride-based compound semiconductor films of high quality entails removing impurity on a surface of the substrate, for example as described in Japanese Patent Laying-Open No. 2003-8060.
Furthermore, after the substrate is provided thereon with a thin, gallium nitride-based compound semiconductor film, an electrode is formed on the thin film to supply the thin film with a current. In that case, that surface of the thin film on which the electrode is deposited must be cleaned to provide closer contact between the electrode and the thin film and provide a device with an improved characteristic. Accordingly, before the electrode is formed, the thin film can have the surface cleaned.
Typically, semiconductor is cleaned with an organic solvent, pure water, an acidic solution, an alkaline aqueous solution or the like, or a combination of any thereof. Si and similar, conventional semiconductors are typically RCA-cleaned, for example as described in “Techniques Employed to Clean Surface of Silicon Wafer, New Edition”, edited by Tsuyoshi Hattori, Realize Co., 2000, p. 394. RCA cleaning is done by alkaline cleaning with a cleaning liquid based on ammonium, pure water cleaning, acidic cleaning employing a hydrochloric acid-containing solution, and hydrofluoric acid cleaning for removing an oxide on a surface, in combination, and is a cleaning method combining the acidic cleaning, the alkaline cleaning and the like together as they specialize in removing different types of fine particles, respectively. Thus RCA cleaning combining the above described, plurality of types of cleaning techniques is intended to remove any impurities. The hydrochloric acid-containing solution can be replaced with a sulfuric acid-containing solution to clean the semiconductor. Furthermore, before RCA cleaning, or to remove an organic matter in a subsequent step, organic cleaning employing an organic solvent is also generally performed.
The above described cleaning techniques can also be applied to clean nitride-based compound semiconductor. The nitride-based compound semiconductor thus cleaned, however, conventionally, disadvantageously had fine particles adhering on a surface thereof. For example, when a surface of a nitride-based compound semiconductor cleaned with an organic solvent to remove an organic matter or with an acidic solution to remove metallic impurity or the like is compared with that of a generally known, silicon or similar substrate similarly cleaned, it has been confirmed that the former has more fine particles adhering thereon than the latter. Furthermore, in some case, the nitride-based compound semiconductor had a surface with a larger, rather than smaller, number of fine particles adhering thereon after it is cleaned than before it is cleaned.
Thus applying conventional cleaning techniques to nitride-based compound semiconductor results in the cleaned semiconductor having a surface with fine particles adhering thereon. As such, it is difficult to exactly apply the conventional cleaning techniques to cleaning a surface of nitride-based compound semiconductor. Cleaning compound semiconductors other than nitride-based compound semiconductor, such as InP, GaAs and the like, is also accompanied by a similar problem.